Method of forming epitaxial buffer layer for finfet source and drain junction leakage reduction

ABSTRACT

A semiconductor device including a gate structure on a channel region portion of a fin structure, and at least one of an epitaxial source region and an epitaxial drain region on a source region portion and a drain region portion of the fin structure. At least one of the epitaxial source region portion and the epitaxial drain region portion include a first concentration doped portion adjacent to the fin structure, and a second concentration doped portion on the first concentration doped portion. The second concentration portion has a greater dopant concentration than the first concentration doped portion. An extension dopant region extending into the channel portion of the fin structure having an abrupt dopant concentration gradient of n-type or p-type dopants of 7 nm per decade or greater.

BACKGROUND

1. Technical Field

The present disclosure relates generally to semiconductor fabrication,and more particularly to structures and methods for forming fin fieldeffect transistors (finFETs).

2. Description of the Related Art

With the continuing trend towards miniaturization of integrated circuits(ICs), there is a need for transistors to have higher drive currentswith increasingly smaller dimensions. FinFET technology is becoming moreprevalent as device size continues to shrink. However, the cost ofmanufacturing SOI finFETs can be high.

SUMMARY

In one embodiment, a method of forming a semiconductor device isdisclosed that includes forming a fin structure from a bulksemiconductor substrate. The upper surface of the fin structure isprovided by an upper surface of the bulk semiconductor substrate and alength of a sidewall of the fin structure extends from the upper surfaceof the fin structure to a recessed surface of the bulk semiconductorsubstrate present at a base of the fin structure. An undoped epitaxialsemiconductor material is formed on the fin structure. A first portionof undoped epitaxial semiconductor material is formed on the sidewall ofat least one of a source region portion and a drain region portion ofthe fin structure. A second portion of the undoped epitaxialsemiconductor material is formed on the recessed surface of the bulksemiconductor substrate that is present at the base of the finstructure. A doped epitaxial semiconductor material is formed on theundoped epitaxial semiconductor material. The undoped epitaxialsemiconductor material and the doped epitaxial semiconductor materialprovide a source region and drain region to the semiconductor deviceincluding the fin structure.

In another embodiment, the method of forming the semiconductor deviceincludes forming a plurality of fin structures from a bulk semiconductorsubstrate. The length of a sidewall for each fin structure of theplurality of fin structures extends from the upper surface of the finstructure to a recessed surface of a bulk semiconductor substratepresent between adjacent fin structures of the plurality of finstructures. An undoped epitaxial semiconductor material is formed on theplurality of fin structures. A first portion of undoped epitaxialsemiconductor material is formed on at least a portion of the sidewallof a source region portion and a drain region portion of the finstructures. A second portion of the undoped epitaxial semiconductormaterial is formed on the recessed surface of the bulk semiconductorsubstrate that is present between the adjacent fin structures. A dopedepitaxial semiconductor material is formed on the undoped epitaxialsemiconductor material. The undoped epitaxial semiconductor material andthe doped epitaxial semiconductor material provide a merged sourceregion and a merged drain region to the adjacent fin structures of thesemiconductor device.

In yet another aspect, a semiconductor device is provided that includesa gate structure on a channel region portion of a fin structure. Atleast one of an epitaxial source region and an epitaxial drain regionare present on a source region portion and a drain region portion of thefin structure. At least one of the epitaxial source region portion andthe epitaxial drain region portion include a first concentration dopedportion adjacent to the fin structure, and a second concentration dopedportion on the first concentration doped portion. The secondconcentration portion has a greater dopant concentration than the firstconcentration doped portion. An extension dopant region extends from thesource region portion and the drain region portion of the fin structureinto the channel portion of the fin structure having an abrupt dopantconcentration gradient of n-type or p-type dopants of 7 nm per decade orgreater.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a perspective view depicting forming a replacement gatestructure on a plurality of fin structures that are formed from a bulksemiconductor substrate, in accordance with one embodiment of thepresent disclosure.

FIG. 2 is a side cross-sectional view along section line A-A (crosssection perpendicular to the fin structures) of the structure depictedin FIG. 1.

FIG. 3 is a side cross-sectional view perpendicular to the finstructures of forming an undoped epitaxial semiconductor material on thefin structure, in accordance with one embodiment of the presentdisclosure.

FIG. 4 is a side cross-sectional view perpendicular to the finstructures of forming a doped epitaxial semiconductor material on theundoped epitaxial semiconductor material, in accordance with oneembodiment of the present disclosure.

FIG. 5 is a side cross-sectional view perpendicular to the finstructures depicting an epitaxial semiconductor cap being formed on thedoped epitaxial semiconductor material, in accordance with oneembodiment of the present disclosure.

FIG. 6 is a perspective view depicting substituting a functional gatestructure for the replacement gate structure, in accordance with oneembodiment of the present disclosure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Detailed embodiments of the claimed structures and methods are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely illustrative of the claimed structures and methods that maybe embodied in various forms. In addition, each of the examples given inconnection with the various embodiments are intended to be illustrative,and not restrictive. Further, the figures are not necessarily to scale,some features may be exaggerated to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the methods and structures of the present disclosure. Forpurposes of the description hereinafter, the terms “upper”, “lower”,“right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, andderivatives thereof shall relate to the embodiments of the disclosure,as it is oriented in the drawing figures. The term “positioned on” meansthat a first element, such as a first structure, is present on a secondelement, such as a second structure, wherein intervening elements, suchas an interface structure, e.g. interface layer, may be present betweenthe first element and the second element. The term “direct contact”means that a first element, such as a first structure, and a secondelement, such as a second structure, are connected without anyintermediary conducting, insulating or semiconductor layers at theinterface of the two elements.

It has been determined that in finFET semiconductor devices formed onbulk semiconductor substrates that source and drain region punch throughleakage and junction leakage have become a critical concern. In someembodiments, performing a fin recess etch at the source and drain regionportions of the fin structures prior to epitaxial growth of thesemiconductor material for the source and drain regions is preferred toprovide a uniform junction extending from the top of the fin structureto the base of the fin structure. Typically, a sharp, i.e., abrupt,junction profile is preferred at the source and drain region portions ofthe fin structure to a channel region portion of the fin structure toprovide a balance of reduced short channel effects (SCE) and lowextension region resistance (Rext). However, a sharp dopant profile atthe base of the fin structure can give rise to junction leakage.

In some embodiments, the methods and structures disclosed herein form abuffer layer, i.e., undoped epitaxial semiconductor layer, on thesidewalls of the source region portion and the drain region portion offin structures formed from bulk semiconductor substrates to reducesource and drain junction leakage. As used herein, the term “finstructure” refers to a semiconductor material, which can be employed asthe body of a semiconductor device, in which the gate structure ispositioned around the fin structure such that charge flows down thechannel on the two sidewalls of the fin structure and optionally alongthe top surface of the fin structure. A “bulk semiconductor substrate”is a substrate that is composed of a single semiconductor material. Thisis distinguished from a semiconductor on insulator (SOI) substrate thatincludes a buried dielectric layer underlying an upper semiconductorlayer, i.e., semiconductor on insulator (SOI) substrate. In someembodiments, the methods and structures disclosed herein provide abuffer layer having a thin sidewall portion on the sidewalls of thesource and drain region portions of the fin structure and a thick baseportion between the adjacent fin structures. The buffer layer may beundoped, i.e., have no p-type or n-type dopant, or the buffer layer maybe lightly doped, i.e., have a dopant concentration of n-type or p-typedopants that is no greater than 1×10¹⁹ atoms/cm³ to 5×10¹⁹ atoms/cm³.The methods and structures of the present disclosure are now discussedwith more detail referring to FIGS. 1-6.

FIGS. 1 and 2 depict a replacement gate structure 10 that is present ona plurality of fin structures 5 that have been formed from a bulksemiconductor substrate 1. The semiconductor material that provides thebulk semiconductor substrate 1 may be a semiconducting materialincluding, but not limited to silicon, strained silicon, a siliconcarbon alloy (e.g., silicon doped with carbon (Si:C), silicon germanium,a silicon germanium and carbon alloy (e.g., silicon germanium doped withcarbon (SiGe:C), silicon alloys, germanium, germanium alloys, galliumarsenic, indium arsenic, indium phosphide, as well as other III/V andII/VI compound semiconductors.

The plurality of fin structures 5 may be formed from the bulksemiconductor substrate 1 using photolithography and etch processes. Forexample, prior to etching the bulk semiconductor substrate 1 to providethe plurality of fin structures 5, a layer of the dielectric material isdeposited atop the upper surface of the bulk semiconductor substrate 1to provide a dielectric fin cap 3. The material layer that provides thedielectric fin cap 3 may be composed of a nitride, oxide, oxynitridematerial, and/or any other suitable dielectric layer. The dielectric fincap 3 may comprise a single layer of dielectric material or multiplelayers of dielectric materials. The material layer that provides thedielectric fin cap 3 can be formed by a deposition process, such aschemical vapor deposition (CVD) and/or atomic layer deposition (ALD).Alternatively, the material layer that provides the dielectric fin cap 3may be formed using a growth process, such as thermal oxidation orthermal nitridation. The material layer that provides the dielectric fincap 3 may have a thickness ranging from 1 nm to 100 nm. In one example,the dielectric fin cap 3 is composed of an oxide, such as SiO₂, that isformed by CVD to a thickness ranging from 25 nm to 50 nm.

In one embodiment, following the formation of the layer of dielectricmaterial that provides the dielectric fin cap 3, a photolithography andetch process sequence is applied to the material layer for thedielectric fin cap 3 and the bulk semiconductor substrate 1.Specifically, in one example, a photoresist mask is formed overlying thelayer of the dielectric material that provides dielectric fin cap 3 andis present overlying the bulk semiconductor substrate 1, in which theportion of the dielectric material that is underlying the photoresistmask provides the dielectric fin cap 3, and the portion of the bulksemiconductor substrate 1 that is underlying the photoresist maskprovides the plurality of fin structures 5. The exposed portions of thedielectric material that provides dielectric fin cap 3 and the portionof the bulk semiconductor substrate 1 that are not protected by thephotoresist mask are removed using a selective etch process. To providethe photoresist mask, a photoresist layer is first positioned on thelayer of the dielectric material that provides dielectric fin cap 3. Thephotoresist layer may be provided by a blanket layer of photoresistmaterial that is formed utilizing a deposition process such as, e.g.,plasma enhanced CVD (PECVD), evaporation or spin-on coating. The blanketlayer of photoresist material is then patterned to provide thephotoresist mask utilizing a lithographic process that may includeexposing the photoresist material to a pattern of radiation anddeveloping the exposed photoresist material utilizing a resistdeveloper.

Following the formation of the photoresist mask, an etching process mayremove the unprotected portions of the dielectric material that providesthe dielectric fin cap 3 followed by removing a portion of the exposedbulk semiconductor substrate 1 selectively to the photoresist mask. Forexample, the transferring of the pattern provided by the photoresistinto the underlying structures may include an anisotropic etch. As usedherein, an “anisotropic etch process” denotes a material removal processin which the etch rate in the direction normal to the surface to beetched is greater than in the direction parallel to the surface to beetched. The anisotropic etch may include reactive-ion etching (RIE).Other examples of anisotropic etching that can be used at this point ofthe present disclosure include ion beam etching, plasma etching or laserablation. The etch process may be timed to determine the height of thefin structures 5. In some embodiments, following etching of the bulksemiconductor substrate 1 to define the fin structures 5, the dielectricfin cap 3 may be removed by a selective etch.

The upper surface of the fin structures 5 is provided by an uppersurface of the bulk semiconductor substrate 1 that is under thedielectric fin cap 3 and is therefore not etched. The etched portions ofthe bulk semiconductor substrate 1 are recessed surfaces relative to theupper surface of the fin structure 5. The sidewall of the fin structure5 extends from the recessed surfaces of the bulk semiconductor substrate1 to the upper surface of the fin structure 5 that is provided by anupper surface of the bulk semiconductor substrate 1.

Each of the fin structures 5 may have a height H₁ ranging from 5 nm to200 nm. In another embodiment, each of the fin structures 5 has a heightH₁ ranging from 10 nm to 100 nm. In one example, each of the finstructures 5 has a height H₁ ranging from 20 nm to 50 nm. Each of theplurality of fin structures 5 may have a width W₁ of less than 20 nm. Inanother embodiment, each of the fin structures 5 has a width W₁ rangingfrom 3 nm to 8 nm. Although two fin structures 5 are depicted in FIG. 1,the present disclosure is not limited to only this example. It is notedthat any number of fin structures 5 may be formed from the bulksemiconductor substrate 1. The pitch P1 separating adjacent finstructures 5 may range from 35 nm to 45 nm. In another example, thepitch P1 separating adjacent fin structures 5 may range from 30 nm to 40nm.

FIGS. 1 and 2 further depict forming a replacement gate structure 10 onthe channel portion of the fin structures 5. As used herein, the term“replacement gate structure” denotes a sacrificial structure thatdictates the geometry and location of the later formed functioning gatestructure. The “functional gate structure” functions to switch thesemiconductor device from an “on” to “off” state, and vice versa. In oneembodiment, the replacement gate structure 10 includes a sacrificialgate material 11, and a sacrificial gate cap 12.

In one embodiment, the sacrificial gate material 11 of the replacementgate structure 10 may be composed of any material that can be etchedselectively to the fin structures 5 and the isolation region 15. As usedherein, the term “selective” in reference to a material removal processdenotes that the rate of material removal for a first material isgreater than the rate of removal for at least another material of thestructure to which the material removal process is being applied. Forexample, in one embodiment, a selective etch may include an etchchemistry that removes a first material selectively to a second materialby a ratio of 100:1 or greater, e.g., 1000:1.

In one embodiment, the sacrificial gate material 11 of the replacementgate structure 10 may be composed of a silicon-including material, suchas polysilicon. In one embodiment, the sacrificial gate cap 12 may becomposed of a dielectric material, such as an oxide, nitride oroxynitride material, or amorphous carbon. The sacrificial materials thatprovide the sacrificial gate material 11 and the sacrificial gate cap 12may be patterned and etched to provide the replacement gate structure10. It is noted that the replacement gate structure 10 is not limited toonly the example that is depicted in FIGS. 1 and 2. For example, thereplacement gate structure 10 may be composed of any number of materiallayers and any number of material compositions, so long as thesacrificial gate material 11 can be removed selectively to the pluralityof fin structures 5.

Referring to FIGS. 1 and 2, in some embodiments a gate sidewall spacer13 is formed on the sidewall of the replacement gate structure 10. Inone embodiment, the gate sidewall spacer 13 may be formed by using ablanket layer deposition process, such as CVD, and an anisotropicetchback method. The gate sidewall spacer 13 may have a width rangingfrom 2.0 nm to 15.0 nm, and may be composed of a dielectric, such as anitride, oxide, oxynitride, or a combination thereof.

FIG. 3 depicts one embodiment of forming an undoped epitaxialsemiconductor material 6 a, 6 b on the fin structure 5. In someembodiments, a first portion 6 a of the undoped epitaxial semiconductormaterial 6 a, 6 b is formed on the sidewall S1 of at least one of asource region portion and a drain region portion of the fin structure 5,and a second portion 6 b of the undoped epitaxial semiconductor material6 a, 6 b is formed on the recessed surface S2 of the bulk semiconductorsubstrate 1 that is present at the base of the fin structure 5.

In one embodiment, the undoped epitaxial semiconductor material 6 a, 6 bis formed on the exposed section of the sidewall S1 of the source regionportion and the drain region portion of the fin structure 5 and therecessed surface S2 of the bulk semiconductor substrate 1 using anepitaxial deposition process. “Epitaxial growth and/or deposition” meansthe growth of a semiconductor material on a deposition surface of asemiconductor material, in which the semiconductor material being grownhas substantially the same crystalline characteristics as thesemiconductor material of the deposition surface. In some embodiments,when the chemical reactants are controlled and the system parameters setcorrectly, the depositing atoms arrive at the deposition surface withsufficient energy to move around on the surface and orient themselves tothe crystal arrangement of the atoms of the deposition surface. Thus, insome examples, an epitaxial film deposited on a {100} crystal surfacewill take on a {100} orientation.

The term “undoped” means that the undoped epitaxial semiconductormaterial 6 a, 6 b is substantially free of p-type or n-type dopant. By“substantially free” of p-type or n-type dopant, it is meant that themaximum concentration of p-type or n-type dopant in the undopedepitaxial semiconductor material 6 a, 6 b is no greater than 5×10¹⁵atoms/cm³. In one embodiment, the concentration of n-type or p-typedopant in the undoped epitaxial semiconductor material 6 a, 6 b mayrange from 1×10¹⁵ atoms/cm³ to 5×10¹⁵ atoms/cm³. In yet anotherembodiment, the concentration of n-type or p-type dopant in the undopedepitaxial semiconductor material 6 a, 6 b may be less than 1×10¹⁵atoms/cm³. In one example, the undoped epitaxial semiconductor material6 a, 6 b is entirely free of p-type or n-type dopants.

In some embodiments, epitaxial deposition of the undoped epitaxialsemiconductor material 6 a, 6 b is a selective deposition process. Forexample, although the epitaxially deposited the undoped epitaxialsemiconductor material 6 a, 6 b orientates to the crystal arrangement ofa semiconductor material and is deposited thereon, such as the exposedsidewall surface S1 of the fin structures 5, the epitaxially depositedundoped epitaxial semiconductor material 6 a, 6 b may not be depositedon a dielectric material. For example, the undoped epitaxialsemiconductor material 6 a, 6 b is not formed on the sacrificial gatecap 12, the dielectric fin cap 3 and the gate sidewall spacer 13.

In another embodiment, during the epitaxial deposition of the undopedepitaxial semiconductor material 6 a, 6 b on the fin structures 5,amorphous semiconductor material is deposited on dielectric surfaces,such as the sacrificial gate cap 12, the dielectric fin cap 3 and thegate sidewall spacer 13. The amorphous semiconductor material that isformed on the dielectric surfaces may be removed selectively, e.g.,selectively etched, to the undoped epitaxial semiconductor material 6 a,6 b formed on the exposed sidewalls of the fin structures 5.

In some embodiments, the undoped epitaxial semiconductor material 6 a, 6b may be composed of a germanium-containing semiconductor. Thegermanium-containing semiconductor of the undoped epitaxialsemiconductor material 6 a, 6 b may include greater than 20 at. %germanium. In some embodiments, the germanium-containing semiconductorof the undoped epitaxial semiconductor material 6 a, 6 b may have agermanium content that is 30 at. % or greater. In some examples, thegermanium content of the germanium-containing semiconductor may rangefrom 35 at. % to 55 at. %. In another example, the germanium content ofthe germanium-containing semiconductor may range from 40 at. % to 50 at.%. In one embodiment, the undoped epitaxial semiconductor material 6 a,6 b is composed of germanium, silicon germanium, a silicon germanium andcarbon alloy (e.g., silicon germanium doped with carbon (SiGe:C),germanium alloys, gallium arsenic, indium arsenic, indium phosphide, aswell as other III/V and II/VI compound semiconductors.

In one embodiment, a number of different sources may be used for theepitaxial deposition of the undoped epitaxial semiconductor material 6a, 6 b. Examples of germanium including source gasses for epitaxiallyforming an undoped epitaxial semiconductor material 6 a, 6 b of agermanium containing semiconductor include germane, digermane,halogermane, dichlorogermane, trichlorogermane, tetrachlorogermane andcombinations thereof. The germanium including source gas may beaccompanied by a silicon including source gas. Examples of siliconincluding source gasses may include silane, disilane, trisilane,tetrasilane, hexachlorodisilane, tetrachlorosilane, dichlorosilane,trichlorosilane, methylsilane, dimethylsilane, ethylsilane,methyldisilane, dimethyldisilane, hexamethyldisilane and combinationsthereof.

Epitaxial deposition may be carried out in a chemical vapor depostionapparatus, such as a metal organic chemical vapor deposition (MOCVD)apparatus or a plasma enhanced chemical vapor deposition (PECVD)apparatus. The temperature for epitaxial deposition typically rangesfrom 550° C. to 900° C. Although higher temperature typically results infaster deposition, the faster deposition may result in crystal defectsand film cracking. In some embodiments, the epitaxial deposition processmay be adjusted so that the first portion 6 a of the undoped epitaxialsemiconductor material 6 a, 6 b that is formed on the sidewall S1 of atleast one of a source region portion and a drain region portion of thefin structure 5 has a first thickness T1, and the second portion 6 b ofthe undoped epitaxial semiconductor material 6 a, 6 b that is formed onthe recessed surface S2 of the bulk semiconductor substrate 1 that ispresent at the base of the fin structure 5 has a second thickness T2that is greater than the first thickness T1.

In one embodiment, the first thickness T1 of the first portion 6 a ofthe undoped epitaxial semiconductor material 6 a, 6 b may range from 2nm to 6 nm, and the second thickness T2 of the second portion 6 b of theundoped epitaxial semiconductor material 6 a, 6 b may range from 7 nm to12 nm. In another embodiment, the first thickness T1 of the firstportion 6 a of the undoped epitaxial semiconductor material 6 a, 6 b mayrange from 3 nm to 5 nm, and the second thickness T2 of the secondportion 6 b of the undoped epitaxial semiconductor material 6 a, 6 b mayrange from 8 nm to 10 nm.

In one example, the source gas and vacuum pressure applied during theepitaxial deposition process is selected so that the portion of theepitaxially deposited material, i.e., the first portion 6 a of undopedepitaxial semiconductor material 6 a, 6 b, deposited on the sidewalls S1of the fin structures 5 has a lesser thickness than the portion of theepitaxial deposited material, i.e., the second portion 6 b of theundoped epitaxially deposited material 6 a, 6 b, that is formed on therecessed surfaces S2 of the bulk semiconductor substrate 1. For example,to provide a greater thickness of epitaxially deposited material on thehorizontal surfaces of the bulk semiconductor substrate 1 between theadjacent fin structures 10, than the epitaxially deposited material onthe vertical sidewall surfaces S1 of the fin structures 10, the gassource may be selected to be dichlorosilane (DCS) or tetrasilane (SiH₄),in which the pressure of the deposition process is equal to 5 Torr orless. In some embodiments, with DCS based epitaxial process whenreducing the chamber pressure it may help to achieve asymmetricalthickness on the fin sidewall (thin) and bottom (thick). Even higherdelta, i.e., asymmetrical thickness on the fin sidewall (thin) andbottom (thick), may be achieved with a change from DCS to SiH₄ based gasthat is processed at high pressure environment, e.g., around 5 Torr.

FIG. 4 depicts one embodiment of forming a doped epitaxial semiconductormaterial 7 on the undoped epitaxial semiconductor material 6 a, 6 b. By“doped” it is meant that the doped epitaxial semiconductor material 7includes a p-type or n-type dopant present therein. For example, theconcentration of the p-type or n-type dopant in the doped epitaxialsemiconductor material 7 may range from 5×10²⁰ atoms/cm³ to 8×10²⁰atoms/cm³. In another example, the concentration of the p-type or n-typedopant in the doped epitaxial semiconductor material 7 may range from6×10²⁰ atoms/cm³ to 7×10²⁰ atoms/cm³.

In the embodiments in which the finFET device being formed has n-typesource and drain regions, and is referred to as an n-type finFET, thedoped epitaxial semiconductor material 7 is doped with an n-type dopantto have an n-type conductivity. In the embodiments in which the finFETdevice being formed has p-type source and drain regions, and is referredto as a p-type finFET, the doped epitaxial semiconductor material 7 isdoped with a p-type dopant to have a p-type conductivity. As usedherein, “p-type” refers to the addition of impurities to an intrinsicsemiconductor that creates deficiencies of valence electrons. In a typeIV semiconductor, such as silicon, examples of p-type dopants, i.e.,impurities, include but are not limited to, boron, aluminum, gallium andindium. As used herein, “n-type” refers to the addition of impuritiesthat contributes free electrons to an intrinsic semiconductor. In a typeIV semiconductor, such as silicon, examples of n-type dopants, i.e.,impurities, include but are not limited to antimony, arsenic andphosphorous.

The doped epitaxial semiconductor material 7 may be composed of agermanium-containing semiconductor. The germanium-containingsemiconductor of the doped epitaxial semiconductor material 7 mayinclude greater than 20 at. % germanium. In some embodiments, thegermanium-containing semiconductor of the doped epitaxial semiconductormaterial 7 may have a germanium content that is 30 at. % or greater. Insome examples, the germanium content of the germanium-containingsemiconductor may range from 25 at. % to 55 at. %. In another example,the germanium content of the germanium-containing semiconductor mayrange from 30 at. % to 50 at. %. In one embodiment, the doped epitaxialsemiconductor material 7 is composed of germanium, silicon germanium, asilicon germanium and carbon alloy (e.g., silicon germanium doped withcarbon (SiGe:C), germanium alloys, gallium arsenic, indium arsenic,indium phosphide, as well as other III/V and II/VI compoundsemiconductors.

The doped epitaxial semiconductor material 7 may be formed using anepitaxial deposition process similar to the epitaxial deposition processfor forming the undoped epitaxial semiconductor material 6 a, 6 b thatis depicted in FIG. 3. One difference between the epitaxial depositionmethod for forming the doped epitaxial semiconductor material 7 from themethod for forming the undoped epitaxial semiconductor material 6 a, 6 bis that in forming the doped epitaxial semiconductor material 7 a dopantis introduced to the epitaxially deposited material by an in situ dopingprocess. The term “in situ” denotes that the dopant, e.g., n-type orp-type dopant, is introduced to the base semiconductor material, e.g.,silicon or silicon germanium, during the formation of the base material.For example, an in situ doped epitaxial semiconductor material mayintroduce n-type or p-type dopants to the material being formed duringthe epitaxial deposition process that includes n-type or p-type sourcegasses.

In one embodiment, the n-type gas dopant source may include arsine(AsH₃), phosphine (PH₃) and alkylphosphines, such as with the empiricalformula R_(x)PH_((3-x)), where R=methyl, ethyl, propyl or butyl and x=1,2 or 3. Alkylphosphines include trimethylphosphine ((CH₃)₃P),dimethylphosphine ((CH₃)₂PH), triethylphosphine ((CH₃CH₂)₃P) anddiethylphosphine ((CH₃CH₂)₂PH). The p-type gas dopant source may includediborane (B₂H₆). The source gas for the base material may be provided bygermane, digermane, halogermane, silane, disilane, trisilane,tetrasilane, hexachlorodisilane, tetrachlorosilane, dichlorosilane,trichlorosilane, methylsilane, dimethylsilane, ethylsilane,methyldisilane, dimethyldisilane, hexamethyldisilane and combinationsthereof.

FIG. 5 depicts one embodiment of forming an epitaxial semiconductor cap8 on the doped epitaxial semiconductor material 7. In one embodiment,the epitaxial semiconductor cap 8 is composed of a silicon containingmaterial. For example, the epitaxial semiconductor cap 8 may be composedof silicon. In some embodiments, the epitaxial semiconductor cap 8 isdoped to have the same conductivity, i.e., p-type or n-type, as theconductivity of the doped epitaxial semiconductor material 7. In otherembodiments, the epitaxial semiconductor cap 8 may be undoped.

FIG. 6 depicts one embodiment of diffusing dopant from the epitaxialsource and drain region 9 (combination of the undoped epitaxialsemiconductor material 6 a, 6 b, doped epitaxial semiconductor material7 and the epitaxial semiconductor cap 8) into the fin structures 5 toform an extension dopant region extending into the channel portion ofthe fin structure 5 having an abrupt dopant concentration gradient ofn-type or p-type dopants ranging from 5 nm per decade to 10 nm perdecade. In some embodiments, the epitaxial source and drain region 9extends from a first fin structure to an adjacent second fin structure.The abrupt concentration gradient is measured from the exterior sidewallof the gate sidewall spacer 13 in a direction extending into the channelregion of the fin structure 5 from one of the source region portion anddrain region portion of the fin structure 5. In one embodiment, theabrupt dopant concentration gradient of n-type or p-type dopants is 7 nmper decade or greater (i.e., less 7 nm per decade, e.g., 6 nm perdecade). In one embodiment, the abrupt dopant concentration gradient ofn-type or p-type dopants is ranges from 5 nm per decade to 7 nm perdecade.

Typically, a greater abruptness is provided by the methods andstructures disclosed herein than previously possible, because the bufferlayer, i.e., the undoped epitaxial semiconductor material 6 a, 6 b,being composed of a germanium containing material slows the diffusion ofdopants from the doped epitaxial semiconductor material 7 to the finstructures 5. In this manner, a greater concentration of dopant may bepresent in the doped epitaxial semiconductor material 7 withoutresulting in a high level of diffusion into the fin structure 5. Forexample, the fin doping level, which is on the order of 1×10¹⁵atoms/cm², is much lower than the source and drain doping level, whichis on the order of 5×10²⁰ atoms/cm² to 8×10²⁰ atoms/cm². If without thebuffer layer to suppress the diffusion the dopant gradient can geteasily be greater than 10 nm per decade. With the buffer layer in place,the “controlled” diffusion can still provide enough gate to extensionoverlap but without incurring very poor junction gradient.

In some embodiments, the diffusion, i.e., driving, of the dopant fromthe doped epitaxial semiconductor material 7 into the extension regionportions of the fin structures 5 comprises thermal annealing. In oneembodiment, the thermal annealing that diffuses the dopant from thedoped epitaxial semiconductor material 7 into the extension regionportions of the fin structures 5 includes an annealing process selectedfrom the group consisting of rapid thermal annealing (RTA), flash lampannealing, furnace annealing, laser annealing and combinations thereof.In one embodiment, the thermal annealing for driving the dopant, i.e.,p-type or n-type dopant, from doped epitaxial semiconductor material 7into the extension region portions of the fin structures 5 may include atemperature ranging from 800° C. to 1200° C., and a time period rangingfrom 10 milliseconds to 100 seconds.

Following the thermal anneal, the combination of the undoped epitaxialsemiconductor material 6 a, 6 b, doped epitaxial semiconductor material7 and the epitaxial semiconductor cap 8 provide an epitaxial source anddrain region 9. In one embodiment, the dopant concentration of n-type orp-type dopant in the portion of the epitaxial source and drain region 9that is provided by the undoped epitaxial semiconductor material 6 a, 6b ranges from 1×10¹⁵atoms/cm³ to 5×10¹⁵ atoms/cm³. In one embodiment,the dopant concentration of n-type or p-type dopant in the portion ofthe epitaxial source and drain region 9 that is provided by the dopedepitaxial semiconductor material 7 ranges from 1×10¹⁹ atoms/cm³ to5×10¹⁹.

FIG. 6 also depicts substituting a functional gate structure 50 for thereplacement gate structure after the application of the thermal anneal.In one embodiment, the replacement gate structure 10 may be removed by aselective etch. The replacement gate structure 10 may be removed using awet or dry etch process. In one embodiment, the replacement gatestructure 10 is removed by reactive ion etch (RIE). In one example, anetch step for removing the replacement gate structure 10 can include anetch chemistry for removing the sacrificial gate material 11 and thesacrificial gate cap 12 of the sacrificial replacement gate structure 10selective to the fin structures 5, and the gate sidewall spacer 13.

A functional gate structure 50 is formed in the space that is providedby removing the replacement gate structure 10. The functional gatestructure 50 is formed in direct contact with a channel region portionof the fin structures 5. The functional gate structure 50 typicallyincludes at least one gate dielectric layer 51 and at least one gateconductor layer 52. The at least one gate dielectric layer 51 istypically positioned directly on at least the channel portion of the finstructure 5. The at least one gate dielectric layer 51 may be formed bya thermal growth process, such as, e.g., oxidation, nitridation oroxynitridation. The at least one gate dielectric layer 51 may also beformed by a deposition process, such as, e.g., CVD, plasma-assisted CVD,MOCVD, ALD, evaporation, reactive sputtering, chemical solutiondeposition and other like deposition processes. The at least one gatedielectric layer 51 may also be formed utilizing any combination of theabove processes.

The at least one gate dielectric layer 51 may be comprised of aninsulating material having a dielectric constant of about 4.0 orgreater. In another embodiment, the at least one gate dielectric layer51 is comprised of an insulating material having a dielectric constantgreater than 7.0. The dielectric constants mentioned herein are relativeto a vacuum. In one embodiment, the at least one gate dielectric layer51 employed in the present disclosure includes, but is not limited to,an oxide, nitride, oxynitride and/or silicates including metalsilicates, aluminates, titanates and nitrides. In one example, when theat least one gate dielectric layer 51 is comprised of an oxide, theoxide may be selected from the group including, but not limited to,SiO₂, HfO₂, ZrO₂, Al₂O₃, TiO₂, La₂O₃, SrTiO₃, LaAlO₃, Y₂O₃ and mixturethereof. The physical thickness of the at least one gate dielectriclayer 51 may vary, but typically, the at least one gate dielectric layer51 has a thickness from 1 nm to 10 nm. In another embodiment, the atleast one gate dielectric layer 51 has a thickness from 1 nm to 3 nm.

After forming the material layer for the at least one gate dielectriclayer 51, a blanket layer of a conductive material which forms the atleast one gate conductor 52 of functional gate structure 50 is formed onthe at least one gate dielectric 51 utilizing a deposition process, suchas physical vapor deposition (PVD), CVD or evaporation. The conductivematerial may comprise polysilicon, SiGe, a silicide, a metal or ametal-silicon-nitride such as Ta—Si—N. Examples of metals that can beused as the conductive material include, but are not limited to, Al, W,Cu, and Ti or other like conductive metals. The blanket layer ofconductive material may be doped or undoped. If doped, an in-situ dopingdeposition process may be employed. Alternatively, a doped conductivematerial can be formed by deposition, ion implantation and annealing.

In another embodiment, the process sequence for forming the finFETstructure depicted in FIG. 6 is formed using a gate first processsequence, which is not depicted in the supplied figures. The methoddepicted in FIGS. 1-6 is a gate last process that includes forming areplacement gate structure. In another embodiment, a functional gatestructure is formed instead of a replacement gate structure, and thefunctional gate structure remains throughout the formation of the finstructure. This is referred to as a gate first process sequence. Bothgate first and gate last process sequences are applicable to the presentdisclosure.

While the present disclosure has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present disclosure. It is therefore intended that the presentdisclosure not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

What is claimed is:
 1. A method of forming a semiconductor devicecomprising: forming a fin structure from a semiconductor substrate,wherein an upper surface of the fin structure is provided by an uppersurface of the semiconductor substrate and a length of a sidewall of thefin structure extends from the upper surface of the fin structure to arecessed surface of the semiconductor substrate present at a base of thefin structure; forming an undoped semiconductor material on a finstructure, wherein a portion of the undoped semiconductor material isformed on a recessed surface of a semiconductor substrate that ispresent at the base of the fin structure; and epitaxially growing adoped semiconductor material on the undoped semiconductor material. 2.The method of claim 1, further comprising forming a gate structure on achannel region portion of the fin structure that is between the sourceregion portion and the drain region portion of the fin structure.
 3. Themethod of claim 2, wherein the gate structure is a replacement gatestructure comprised of a sacrificial material, wherein the replacementgate structure is removed after diffusing dopant from the dopedsemiconductor material to the source region portion and the drain regionportion of the fin structures; and a functional gate is formed in theplace of the replacement gate structure.
 4. The method of claim 1,wherein the forming of the undoped semiconductor material comprisesdepositing a first thickness of the undoped semiconductor material on anentirety of a sidewall of the source region portion and the drain regionportion of the fin structures, and depositing a second thickness of theundoped semiconductor material on the recessed surface of thesemiconductor substrate, wherein the second thickness is greater thanthe first thickness.
 5. The method of claim 1, wherein the forming ofthe doped epitaxial semiconductor material on the undoped semiconductormaterial comprises depositing an in situ doped n-type or p-typesemiconductor material.
 6. The method of claim 5, wherein an n-typedopant in the in situ doped n-type semiconductor material is present ina concentration ranging from 5×10²⁰ atoms/cm³ to 8×10²⁰ atoms/cm³. 7.The method of claim 5, wherein a p-type dopant in the in situ dopedp-type semiconductor material is present in a concentration ranging from5×10²⁰ atoms/cm³ to 8×10²⁰ atoms/cm³.
 8. The method of claim 1 furthercomprising applying a thermal anneal to drive n-type or p-type dopantfrom the doped epitaxial semiconductor material to the source regionportion and the drain region portion of the fin structure.
 9. A methodof forming a semiconductor device comprising: forming an undopedsemiconductor material on a plurality of fin structures, wherein a firstportion of undoped semiconductor material is formed on at least aportion of the sidewall a source region portion and a drain regionportion of the plurality of fin structures, and a second portion of theundoped semiconductor material is formed on a recessed surface of asemiconductor substrate that is present between the adjacent structures;and depositing a doped semiconductor material on the undopedsemiconductor material, wherein the undoped semiconductor material andthe doped semiconductor material provide at least one of a merged sourceregion and a merged drain region.
 10. The method of claim 9 furthercomprising forming a gate structure on a channel region portion of theplurality of fin structures that is between the source region portionand the drain region portion of the plurality of fin structures.
 11. Themethod of claim 10, wherein the gate structure is formed prior toforming the undoped semiconductor material.
 12. The method of claim 10,wherein the gate structure is a replacement gate structure comprised ofa sacrificial material, wherein the replacement gate structure isremoved after diffusing dopant from the doped semiconductor material tothe source region portion and the drain region portion of the pluralityof fin structures; and a functional gate is formed in the place of thereplacement gate structure.
 13. The method of claim 9, wherein theforming of the undoped semiconductor material comprises depositing afirst thickness of the undoped semiconductor material on an entirety ofthe sidewall of the source region portion and the drain region portionof the plurality of fin structures, and depositing a second thickness ofthe undoped epitaxial semiconductor material on the recessed surface ofthe semiconductor substrate present between adjacent fin structures ofthe plurality of fin structures
 14. The method of claim 13, wherein thesecond thickness is greater than the first thickness.
 15. The method ofclaim 9, wherein the depositing of the doped semiconductor material onthe undoped semiconductor material comprises depositing an in situ dopedn-type semiconductor material.
 16. The method of claim 9, wherein theforming of the doped epitaxial semiconductor material on the undopedepitaxial semiconductor material comprises depositing an in situ dopedp-type semiconductor material.
 17. The method of claim 15, wherein ann-type dopant in the in situ doped n-type semiconductor material ispresent in a concentration ranging from 5×10²⁰ atoms/cm³ to 8×10²⁰atoms/cm³.
 18. The method of claim 16, wherein a p-type dopant in the insitu doped p-type semiconductor material is present in a concentrationranging from 5×10²⁰ atoms/cm³ to 8×10²⁰ atoms/cm³.
 19. The method ofclaim 9 further comprising applying a thermal anneal to drive the n-typeor p-type dopant from the doped semiconductor material to the sourceregion portion and the drain region portion of the fin structures. 20.The method of claim 19, wherein the thermal anneal comprises atemperature ranging from 800° C. to 1200° C.